[Agents extension] Detect agent manifest at cwd

[trangevi]

On init, detect if there's a manifest in the current working directory and ask if the user wants to use that one.

[Copilot]

Pull request overview

Adds an azd ai agent init enhancement in the azure.ai.agents extension to auto-detect an agent manifest in the working/source directory and optionally use it, improving the “init from existing files” UX.

Changes:

• Auto-detect agent.manifest.yaml / agent.yaml in the target directory when --manifest is not provided, with a confirmation prompt.
• Add detectLocalManifest + validation helper to ensure detected files are valid agent manifests.
• Add unit tests for local manifest detection; add an overwrite-confirmation when generating agent.yaml from code.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
cli/azd/extensions/azure.ai.agents/internal/cmd/init.go	Adds manifest auto-detection and an interactive “use it?” prompt before selecting init flow.
cli/azd/extensions/azure.ai.agents/internal/cmd/init_from_templates_helpers.go	Introduces detectLocalManifest + manifest validation helper used by init auto-detection.
cli/azd/extensions/azure.ai.agents/internal/cmd/init_from_templates_helpers_test.go	Adds tests covering detection behavior and preference/validation rules.
cli/azd/extensions/azure.ai.agents/internal/cmd/init_from_code.go	Prompts before overwriting an existing agent.yaml when scaffolding from code.

[jongio]

Two features: manifest auto-detection at CWD and overwrite confirmation in init-from-code. Detection logic is solid with content validation and good test coverage (9 cases).

Issues to address:

• init_from_code.go:88 - overwrite check runs after the full interactive setup; declining discards the user's work
• init_from_templates_helpers.go:271 - stat errors other than "not found" silently treated as no manifest

Minor: detectLocalManifest uses exact filename matching while findAgentManifest uses case-insensitive - consider aligning.